The formation of cysts in the kidney is common to a number of disorders in humans, the most prominent of which is polycystic kidney disease (PKD). Several genes associated with PKD have been cloned and the corresponding proteins have been localized to a number of different subcellular domains; however, recent data has raised the possibility that their localization in cilia is important for normal renal physiology and for cilia function. Cilia are highly conserved organelles found on many different cell types in mammals as well as lower eukaryotic organisms where they are involved in a wide range of functions from fluid and cell movement to sensory perception and developmental patterning. In spite of these important roles, very little research has been conducted on how proteins become localized to this structure. This issue is becoming increasingly important with regards to normal tissue physiology, embryogenesis, and disease as evidenced by the developmental defects and severe pathologies seen in mice lacking normal cilia, and the recent localization of a number of proteins to cilia that are associated with human disorders such as PKD and cystic fibrosis. In light of these findings, this proposal centers on two major themes, which are to elucidate the mechanism by which proteins become targeted to cilia and to determine whether cilia dysfunction is a key contributing factor to the formation of cysts in the mammalian kidney. As a consequence of the high degree of evolutionary conservation of cilia, the approaches used in this application involve a combination of in vivo studies in mice and C. elegans as well as in vitro experiments using a cell culture system. In the first aim of this proposal, critical domains required for cilia localization will be identified within two cystic kidney disease related proteins that are found in the cilia of both C. elegans and mice. The goal of the second aim is to elucidate how these proteins are transported from their site of synthesis in the cytosol to their location in the cilia where they function. The third aim will explore whether two recently identified proteins involved in a non-PKD form of renal cystic disease in humans are also present in cilia, thus evaluating the universal nature of the association between cilia and renal cystogenesis. Finally, the fourth aim will directly evaluate the importance of cilia in the etiology of renal cystic disease using a series of conditional mutations in mice that specifically disrupt cilia formation in the kidney. These conditional mutant mice have additional benefits in that they can be used to evaluate the importance of cilia in other pathologies associated with cystic kidney disease as well as provide critical reagents, such as conditional cell lines, that will allow us to further study the role of cilia in normal physiology.